With rapid development of electronic and information industries, a diversity of office machines such as printers, faxing machines or scanners become essential electronic devices in the offices.
Among these office machines, image scanning apparatuses with scanning functions or multifunction peripherals are more popular become they are indispensable to office automation. As known, an image scanning apparatus usually has an automatic document feeder for successively and continuously feeding a stack of paper sheets into the inner portion of the image scanning apparatus. For example, a conventional automatic document feeder has been disclosed in U.S. Pat. No. 6,792,241, which is entitled “Sheet feeding apparatus, image scanning apparatus and image forming apparatus provided with the image reading apparatus”. FIGS. 1A and 1B are schematic views illustrating an automatic document feeder disclosed in U.S. Pat. No. 6,792,241. As shown in FIG. 1A, the automatic document feeder 1 comprises a sheet pick-up arm 10, a stopper 20 and a sheet feeding path C. With the cooperation of the sheet pick-up arm 10 and the stopper 20, the automatic document feeder 1 may pick up an uppermost sheet from a stack of sheets and feed the uppermost sheet into the sheet feeding path C. In addition, the sheet pick-up arm 10 comprises a pressing piece 10a and a sheet pick-up roller 3. The stopper 20 comprises a pressing portion 20a and a stopping surface 20b. 
The sheet pick-up arm 10 is used for feeding the sheets S into the sheet feeding path C of the automatic document feeder 1. In a case that no sheet-feeding operation is done, the sheet pick-up arm 10 is distant from the sheet feeding path C and thus the sheet pick-up arm 10 is in a standby position (see FIG. 1A). For a purpose of feeding the sheets S, the sheet pick-up arm 10 is lowered to a sheet feeding position (see FIG. 1B).
Please refer to FIG. 1A again. When the sheet pick-up arm 10 is lifted to the standby position, the stopper 20 that is arranged in the sheet feeding path C is rotated to be upright according to the gravity of the sheet pick-up arm 10. As such, the stopping surface 20b is vertical to sheet feeding path C to stop the sheets S from entering the sheet feeding path C.
Please refer to FIG. 1B again. When the sheet pick-up arm 10 is lowered to the sheet feeding position, the sheet pick-up roller 3 that is arranged at the front end of the sheet pick-up arm 10 is in contact with the sheets S. Meanwhile, the pressing portion 20a of the stopper 20 is pressed by the pressing piece 10a of the sheet pick-up arm 10, and thus the stopping surface 20b is lowered to the position under the sheet feeding path C. Under this circumstance, the sheets S may be fed into the sheet feeding path C without being stopped by the stopper 20. In other words, the uppermost sheet is allowed to be fed into the inner portion of the automatic document feeder 1.
Nowadays, with increasing awareness of environmental protection, more and more electrical apparatuses are designed in views of power-saving concepts. For achieving a power-saving purpose, an electrical apparatus is operated in a sleeping mode if the electrical apparatus is not used. The office machine with an automatic document feeder has the similar power-saving functions. In a case that the automatic document feeder is in a sleeping mode, no electricity is supplied to the sheet pick-up module. Due to interruption of electricity, the sheet pick-up arm 10 fails to continuously stay in the standby position. In other words, the sheet pick-up arm 10 will be lowered to the sheet feeding position in the sleeping mode. Under this circumstance, the sheet stopping function of the stopper 20 is disabled. If the user forgets that the automatic document feeder is in the sleeping mode but forces a stack of sheets to be introduced into the automatic document feeder, many sheets may be erroneously inserted into the sheet feeding path C. After the automatic document feeder is powered on and the automatic document feeder is in a working mode, the sheet pick-up roller 3 that is arranged at the front end of the sheet pick-up arm 10 will be in contact with the too many sheets. Since too many sheets are simultaneously fed into the internal portion of the sheet feeding path C, a so-called double feeding problem occurs. Meanwhile, since too many sheets are jammed in the sheet feeding path C, the automatic document feeder fails to be normally operated.